A cosmetic having ultraviolet shielding ability comprises an organic or inorganic material having ultraviolet shielding ability. The organic materials have a problem in safety due to degradability and, accordingly, inorganic materials are often being used at present. The inorganic material commonly used is a metal oxide. In particular, use of titania powder is widespread and zinc oxide powder is also used.
If titania is selected as an example, there are various titania powders with different particle sizes and it is known that ultraviolet shielding ability of titania powder depends greatly on its primary particle size.
A powder having a large primary particle size (about 200 nm) which is predominantly used as a pigment can exhibit good- shielding effect by scattering but it is not suitable for ultraviolet shielding cosmetics because the ultraviolet shielding ability is low. On the other hand, the fine powder (primary particle size: 10 to 30 nm) exhibits high ultraviolet shielding ability in the short wavelength region (UVB, wavelength: 290 to 320 nm) and can give feeling of transparency but has no shielding effect by the scattering and has a problem that the shielding effect against ultraviolet rays in the long wavelength region (UVA region; wavelength: 320 to 400 nm) is low. Further, titania powder having a primary particle size of about 100 nm can have a good shielding ability against ultraviolet rays in the long wavelength region (UVA), but the titania powder having such a particle size is low in spreadability and when it is incorporated into cosmetics, a problem arises that the feeling on use is uncomfortable.
In order to improve the feeling on use, an inorganic powder such as talc, mica or silica beads or an organic powder such as nylon or polystyrene is blended in some cases, however, these powders have no ultraviolet shielding ability and, on taking account of the ultraviolet shielding ability of cosmetics, the incorporating of these powders is limited.
The metal oxide such as titania and zinc oxide is known to have a photocatalytic activity effect and thus a possibility of adverse affect on the human body and for incorporating it into cosmetics, the metal oxide must be coated with an inorganic coating which is not degenerated by photocatalytic reaction. For this, various surface treated titania powders, for example, titania powders coated with a calcinated alumina coating, coated with a calcinated alumina coating combined with a further surface treatment by stearic acid, glycerol, etc., coated with a calcinated alumina and zirconium oxide coating coated with a calcined silica coating, and the like are commercially available. However, in conventional surface treated metal oxide powders, when incorporated in cosmetics, the coating has a poor effect of shielding photocatalytic activity, so that prevention of phototoxicity may be insufficient and degradation of organic components cannot be prevented. Furthermore, they have a fatal disadvantage in that the feeling, during use of the cosmetics, is not good since the properties of the coating are not appropriate.
The practical silica coating obtained through calcination in the normal sol-gel method generally has an absorption peak intensity ratio I (I=I.sub.1 /I.sub.2, wherein I.sub.1 is an absorption peak intensity at 1,150 to 1,250 cm.sup.-1 and I.sub.2 is an absorption peak intensity at 1,000 to 1,100 cm.sup.-1) between the infrared absorption spectra in the region of 1,150 to 1,250 cm.sup.-1 and the region of 1,000 to 1,100 cm.sup.-1, of less than 0.2. This value I is known to have a tendency in general to become small after calcination. It is also known that due to the calcination, the chemical bond or functional group changes and the silica coating is altered in characteristics such as hydrophilicity or absorption ability of oil. On the other hand, the silica coating obtained without passing through calcination in the normal sol-gel method exhibits a some high absorption peak intensity in the region of from 1,150 to 1,250 cm.sup.-1, however, the refractive index is less than 1.435 and the coating is low in density and is not practical. The density and the refractive index of the silica coating are generally considered to have a positive correlation (see, for example, C. JEFFEREY BRINKER, SOL-GEL SCIENCE, 581-583, ACADEMIC PRESS (1990)).
The first object of the present invention is to provide cosmetics giving excellent feeling on use, having high ultraviolet shielding ability, free of phototoxicity and excellent in storage stability.
The second object of the present invention is to provide a metal oxide powder coated with a dense and practical silica coating having specific characteristics and a good shape-following capability, and a method for producing the silica-coated metal oxide powder.